Morbidity/mortality in severe sepsis are largely due to multiple organ dysfunction/failure, most commonly lung injury, as well as renal and cardiac dysfunction. Despite recent advances in defining many aspects of the pathogenesis of sepsis related multiple organ dysfunction/failure including acute lung injury (ALI) and its severe form acute respiratory distress syndrome, there are currently no effective pharmacological or cell-based treatments of the disease. Evidence from human and animal studies has demonstrated the key role of microvascular leakage in determining the outcome of sepsis. Thus, targeting microvascular leakage repair mechanisms to restore endothelial barrier integrity represents a novel, effective therapeutic approach for treatment of severe sepsis and associated ALI/ARDS. However, little is known about the signaling mechanisms regulating endothelial barrier repair, and hence few crucial druggable targets have been identified. Employing a novel mouse model of tamoxifen-inducible endothelial cell (EC)-specific deletion of Cxcr4 (Cxcr4i?) as well as CXCR4-specific antagonist, our Supporting Data show that inhibition of CXCR4 signaling impairs endothelial barrier repair leading to defective resolution of inflammation. We also observed that EC-expressed p110? isoform of PI3K is the critical signaling component of this repair program. Thus, the proposed studies, employing a multi-discipline strategy including genetic lineage tracing analysis, novel animal models and integrated pharmacological and molecular and cellular biological approaches, will define for the first time the role of CXCR4 p110? signaling in the mechanism of endothelial barrier repair and resolving inflammation and lung injury induced by sepsis. The proposed studies will address the following Specific Aims. In Aim 1, we will determine the role of endothelial p110? signaling in regulating endothelial barrier repair and resolving inflammatory lung injury following sepsis challenge. In Aim 2, we will define endothelial CXCR4 as the critical upstream activator of p110? -dependent endothelial barrier repair. In Aim 3, we will determine the role of EC-expressed SDF-1 in the mechanism of activation of CXCR4 p110? signaling and the clinical potential of activation of this reparative signaling pathway through pharmacological approach and progenitor cell-based SDF-1?herapy of severe sepsis and associated ARDS. We hope through these studies to define the crucial intrinsic reparative mechanisms of endothelial barrier repair and thereby provide novel drug targets for viable and efficacious therapies of severe sepsis and related ARDS.